A Multiobject Double Spectrograph for the Large ... - CiteSeerX

12 downloads 18045 Views 483KB Size Report
significant fraction of our LBT observing time to these programs, the ultimate outcome of which should be ..... expansion (CTE), low cost, and ease of fabrication.
header for SPIE use

A Multi-object Double Spectrograph for the Large Binocular Telescope P. S. Osmer, B. Atwood, P. L. Byard, D. L. DePoy, T. P. O’Brien, R. W. Pogge, D. Weinberg Department of Astronomy, The Ohio State University ABSTRACT We are building a Multi-Object Double Spectrograph for the Large Binocular Telescope. The main themes of our planned research with the instrument are the formation and evolution of galaxies and their nuclei and the evolution of large-scale structure in the universe, although we expect that the spectrograph will be used for many other varieties of programs as well. The science goals for the instrument dictate that it have the highest possible throughput from 320 to 1000 nm, spectral resolutions of 103 to 104, and multi-object capability over an ~6' field. Our design is highly modular, so future upgrades (e.g., additional cameras, new gratings, and integral field units) should be straightforward. Keywords: Optical spectrographs, large telescopes

1. INTRODUCTION The Large Binocular Telescope (LBT) project is a partnership of the University of Arizona, the German LBTB consortium (MPIA, lead institute), the Italian astronomical community (through Arcetri Observatory), the Ohio State University, and the Research Corporation to build the world’s largest telescope on a single mount. The telescope construction is progressing well and first light is expected in 2003. The initial instrument complement of the telescope will include a near-infrared camera/spectrograph (see Mandel et al. and Thatte et al. in these proceedings), a wide field optical imager (see Ragazzoni et al. in these proceedings), some interferometric capability (e.g. Hinz et al. in these proceedings), and an optical spectrograph. As part of our commitment to the project, we are building the optical spectrograph, which will be a facility instrument for all the LBT community. The primary science driver for the instrument for our group is a set of observational programs designed to address several key research topics on the evolution of galaxies and structure in the Universe. We plan to devote a significant fraction of our LBT observing time to these programs, the ultimate outcome of which should be major advances in our understanding of galaxy evolution. Our LBT partners will use the instrument for a wide variety of other research programs as well.

2. RESEARCH PROGRAM Recent increases in the aperture and image quality of ground-based optical telescopes and the sensitivity of their instruments have greatly enhanced their power as cosmic time machines, capable of studying the populations of objects present when the universe was a small fraction of its current age. Unraveling cosmic history by studying the properties of faint, highly redshifted sources and the absorption by intervening material is one of the most compelling challenges for observational astronomy in the next decade. The OSU astronomy department will have a 1/6 share of observing time on the Large Binocular Telescope (LBT). During the first five years of LBT operation, the department plans to devote a substantial fraction (more than 50%) of its observing time to spectroscopic surveys aimed at understanding the formation and evolution of galaxies and active galactic nuclei and the evolution of large scale structure. Furthermore, since the spectrograph will serve as the LBT facility optical spectrograph, the instrument should see extensive service with the other LBT partners for a wide variety of research programs. There are numerous open questions that our observations will address. What is the cosmic history of star formation and chemical enrichment? What physical processes determine this history? When did galaxies of different luminosities and morphologies assemble most of their mass into coherent units? What is the relation between galaxies observed at high redshift and galaxies in the universe today? What are the relations between the populations of high-z quasars, low-z AGNs, and supermassive black holes in local galaxies? What is the typical lifetime of luminous quasars? What mechanisms trigger quasar activity, and what physics drives the turn-on and turn-off of the quasar population? What are the relations between the diverse populations of objects by which we trace the evolution of structure in the universe: galaxies, quasars, damped Lyα systems, Lyman limit systems, and low column density Lyα forest absorbers? How does the structure traced by these populations relate to the structure in the underlying distribution of dark matter?

The last few years have seen major observational advances in these areas, including redshift surveys of flux-limited samples that probe the galaxy distribution out to z≈1 (e.g., Lilly et al. 1995, Lin et al. 1999), HST studies of the morphological evolution of galaxies over this redshift range (e.g., Abraham et al. 1996), and “demographic” studies of the population of supermassive black holes in nearby galaxies (e.g., Magorrian et al. 1998, van der Marel 1999). Most dramatic has been the discovery of a large population of “normal” star-forming galaxies at z>3, through a combination of multi-color selection of “Lyman-break” candidates and spectroscopic confirmation with the LRIS spectrograph on Keck (e.g., Steidel et al. 1996, Lowenthal et al. 1997). More recently, this population has also been probed with Lyα emission-line surveys (e.g., Hu et al. 1998), and discoveries of distant galaxies and quasars are now reaching to z=5 and beyond (Weymann et al. 1998, Spinrad et al. 1998, Chen et al. 1999, and Fan et al. 1999). These developments have made possible the first serious attempts at one of the major objectives of observational cosmology: a determination of the global history of star formation in the universe (e.g., Madau et al. 1996, Madau 1997, and Steidel et al. 1999). However, this determination suffers from many uncertainties, such as the poorly constrained contribution from low-luminosity systems and the possibility, supported by some studies of faint sub-millimeter sources (e.g.,Blain et al 1999), that a large fraction of the star formation occurs in regions enshrouded by dust. Even the basic properties of the Lyman-break objects at z≈3 are a matter of debate. Some argue that these UV-luminous objects are massive systems forming stars at a fairly steady rate (e.g., Steidel et al. 1996), and others that they are small systems whose UV emission has been temporarily boosted by sudden bursts of star formation (e.g., Sawicki & Yee 1998, Kolatt et al. 1999). These disparate points of view have radically different implications for the place of Lyman-break systems in the overall story of galaxy formation and evolution. Alongside the observational breakthroughs have come major advances in the theoretical framework for describing galaxy formation and evolution, with increasing sophistication of semi-analytic models (e.g., Kauffmann et al. 1993; Cole et al. 1994; Somerville & Primack 1999) and hydrodynamic numerical simulations (e.g., Navarro & Steinmetz 1997; Weinberg et al. 1997 and references therein). In contrast to the traditional picture in which galaxies maintain their identity and evolve largely in isolation, theoretical studies of hierarchical galaxy formation suggest that mergers and radical morphological transformations are a common feature of galaxy evolution, and that many of a galaxy’s stars form in sub-units that only later assemble into the galaxy itself. Recent analytic models have begun to explore the connection between the formation of galaxies and the onset and eventual decline of quasar activity (e.g., Haehnelt et al. 1998). Perhaps the most revolutionary theoretical transformation has been the new understanding of the low column density Lyα forest that has emerged from hydrodynamic cosmological simulations (e.g., Cen et al. 1994; Zhang et al. 1995, Hernquist et al. 1996, Miralda-Escudé et al. 1996) and related analytic models (e.g., Bi & Davidsen 1997, Hui et al. 1997). These investigations imply that there is a tight and physically straightforward correlation between observable Lyα optical depth and underlying dark matter density. They also imply that the statistical properties of absorption in Lyα forest spectra with resolution R≈2000-8000 can provide powerful constraints on cosmological models and on the structure of the dark matter distribution (see, e.g., Croft et al. 1999, Nusser & Haehnelt 1999, Weinberg et al. 1999a). We plan to pursue three linked observing programs that would lead to major advances in the understanding of cosmic structure formation and the evolution of the galaxy and quasar populations: 1. 2. 3.

A spectroscopic survey of galaxies with z